Draw border-radius, box-shadow and 3D border styles on every element, blur text-shadow, and draw outline #603

Part of #552 (step 28, missing support, medium priority).

Several border and shadow effects mPDF draws on some elements and not on others:

• border-radius: blocks and images only.
• box-shadow: blocks only.
• groove, ridge, inset, outset: two-tone in tables, flat everywhere else.
• text-shadow: blur is parsed and never drawn.
• outline: drawn nowhere.

Rounded table cards and cells, shadowed tables, outlined boxes and soft text shadows all come out square, flat or missing.

Measured on gravitypdf at 60b82630, PHP 8.5.4, rasterised.

Where each is drawn today.

• Block radius and shadow. Read in setCSS() (src/Mpdf.php:19298-19328) and drawn in PaintDivBB(): the radius clips through RoundedBox::fit() and path() (:17901), and the shadow is drawn from :18303.
• Image radius. Img::open() (src/Tag/Img.php:295-320), and drawn through RoundedBox::imageBox() (src/Mpdf.php:8199-8204).
• Inline borders and backgrounds. Drawn per text run in Cell() (src/Mpdf.php:5788 onwards), with no radius.
• Table and cell borders. The table border code draws groove, ridge, inset and outset in two tones with ColorConverter::lighten() and darken() (src/Mpdf.php:22106-22140). Block borders go through _setBorderLine(), which draws one colour.
• Text shadows. Drawn as offset copies of the text (src/Mpdf.php:6253-6268). The blur that ShadowParser reads (src/Css/ShadowParser.php:225) is never used.
• outline. No box outline is drawn anywhere. Stop reading outline-width and outline-color as the text outline #578 stops outline-width and outline-color stroking the element's text, as mPDF's own text-outline, and leaves drawing a real outline to this issue.

Fix. Each can be its own PR.

• Radius on cells and tables. Clip the cell's background to a rounded path with RoundedBox, and draw the border along it. With border-collapse: collapse, CSS ignores border-radius on cells and on the table, so only the separate model changes.

• Radius on inline elements. Round only the first fragment's left corners and the last fragment's right corners (box-decoration-break: slice).

• Shadows on cells, tables, inline elements and images. Reuse the PaintDivBB() shadow code for a box given its corners and radii.

• 3D styles on blocks, inline elements and images. Reuse the two-tone colours from the table code.

• text-shadow blur. PDF has no blur, so approximate it:

  • draw several copies of the text around the offset, at falling alpha
  • or stroke the text at growing widths and falling alpha

  Both need transparency. Under PDF/A-1 and PDF/X-1a, draw it sharp with a warning, as box-shadow does (transparencyAllowed()).

• outline.

  • Read outline, outline-style, outline-width, outline-color and outline-offset.
  • Draw the outline outside the border box, following the radius. It takes no space in layout.
  • outline-color defaults to currentColor, which currentColor in a border raises warnings and draws black, and a border with no colour is not drawn in the text colour #540 resolves.
  • Draw it on blocks, cells, images and inline elements, after the content. An outline is drawn on top.

Tests. Each row above, a rounded cell under collapse (unchanged), an inline element that wraps onto two lines, shadows with a radius, an outline with outline-offset, and a blurred text-shadow under PDF/A-1b. Snapshots for the drawing, and unit tests for the paths.